The purpose of the proposed research is to investigate human performance in situations of uncertainty and risk. Previous research has found that humans are risk averse when given few choices between certain and probabilistic hypothetical outcomes. These findings appear inconsistent with the many real-world examples of humans risk taking, and with much non-human risky-choice research. Direct comparisons between human and non-human performance are difficult to make, however because the procedures used to study human and non-human risky choice vary substantially from one another. A primary aim of this research is to examine human risky choice under conditions that are more comparable to those used with non-humans to further investigate some of these discrepancies and to provide additional information in areas wherein no comparable data exist. Two series of experiments will investigate how different kinds of outcomes (real vs. hypothetical, amount vs. delays, and gains vs. losses) and changes in one type of motivational variable (energy budget or aspiration level) influence risk taking. These experiments will also evaluate two models of risky choice, prospect theory from cognitive psychology and the energy-budget rule from behavioral ecology. This research, besides being relevant to cognitive, behavior analytic, and ecological models of choice, may reveal processes useful for understanding humans behavior in situations involving health and safety risks as well as gambling and economic issues.